physiology of cerebral blood flow and CSF

?abnormalities of any of these can profoundly affect brain function. For instance, total cessation of blood flow to the brain causes unconsciousness within 5 to 10 seconds. This occurs because lack of oxygen delivery to the brain cells shuts down most metabolism in these cells. Also, on a longer time scale, abnormalities of the cerebrospinal fluid, either its composition or its fluid pressure, can have equally severe effects on brain function.
?Normal blood flow through the brain of the adult person averages 50 to 65 milliliters per 100 grams of brain tissue per minute. For the entire brain, this amounts to 750 to 900 ml/min, or 15 per cent of the resting cardiac output.
?Regulation of Cerebral Blood Flow
? As in most other vascular areas of the body, cerebral blood flow is highly related to metabolism of the tissue. At least three metabolic factors have potent effects in controlling cerebral blood flow: (1) carbon dioxide concentration, (2) hydrogen ion concentration, and (3) oxygen concentration.
?As is true for almost all other tissues of the body, the number of blood capillaries in the brain is greatest where the metabolic needs are greatest. The overall metabolic rate of the brain gray matter where the neuronal cell bodies lie is about four times as great as that of white matter; correspondingly, the number of capillaries and rate of blood flow are also about four times as great in the gray matter.
?An important structural characteristic of the brain capillaries is that they are much less "leaky" than the blood capillaries in almost any other tissue of the body. One reason for this is that the capillaries are supported on all sides by "glial feet," which are small projections from the surrounding glial cells that abut against all surfaces of the capillaries and provide physical support to prevent overstretching of the capillaries in case of high capillary blood pressure.
?The walls of the small arterioles leading to the brain capillaries become greatly thickened in people who develop high blood pressure, and these arterioles remain significantly constricted all the time to prevent transmission of the high pressure to the capillaries.